Soil is the layer of minerals and organic matter reaching 1-2m below the Earth's surface. Its main components are mineral matter, organic matter, soil solution and air. Soils differ in the ratio of these elements, but each is essential for plant growth.

Minerals in soil are obtained from a variety of sources, but the process which delivers the bulk is weathering of rocks.

Weathering is the action of wind, rain, frost, sunlight and biological processes on rocks, which breaks them down in to smaller particles and releases ions such as K+ and Mg2+ in to the soil solution. Some of these ions are taken up by plants, but the majority not left in solution are absorbed through ion exchange[?] by clays such as montmorillonite[?].

When the level of ions is low in the soil an equilibrium process forces ions back in to solution, where they can be used by plants.

However if acid is introduced into soil, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring farmers apply alkalic materials such as slaked-lime.

Although there exists plenty of elements such as nitrogen, potassium and phosphorus necessary for plant growth in soil, very little of this is in a form which plants can use. In processes such as nitrification[?] and mineralisation[?], bacteria and other organisms convert unusable forms (such as NH4+) in to usable forms (such as NO3-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere.

The organic store in soil is made up of plant debris, animal excreta and other decomposing materials. A lot of the carbon compounds react to form humus, which is composed of very large molecules including esters of carboxylic acid, phenolic compounds and derivatives of benzene. Organic materia in soil provides nutrients necessary for plant growth.

The following text is taken from the Household Cyclopedia of 1881, and should be updated and integrated with the above article:

Soils consist mostly of sand, lime, and clay, with certain saline and organic substances in smaller and varying proportions; but the examination of the ashes of plants shows that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash of cultivated plants; and of these the proportions are not by any means immaterial. In general, the soils which are made up of the most various materials are called alluvial; having been formed from the depositions of floods and rivers. Many of them are extremely fertile. Soils consist of two parts; of an organic part, which can readily be burned away when the surface-soil is heated to redness; and of an inorganic part, which remains fixed in the fire, consisting of earthy and saline substances from which, if carbonic acid or any elastic gas be present, it may, however, be driven by the heat. The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in and upon the soil, which have been spread over it by rivers and rains, or which have been added by the industry of man for the purposes of increased fertility.

This organic part varies much in quantity, as well as quality, in different soils. In peaty soils it is very abundant, as well as in some rich, long cultivated lands. In general, it rarely amounts to one-fourth, or 25 per cent. even in our best arable lands. Good wheat soils contain often as little as eight parts in the hundred of organic animal or vegetable matter; oats and rye will grow in a soil containing only 1 1/2 per cent.; and barley when only two or three parts per cent. are present.

The inorganic portion of any given soil, again, is divisible into two portions; that part which is soluble in water, and thus easily taken up by plants, and a much more bulky portion which is insoluble.

Sir Humphry Davy found the following to be the composition of a good productive soil.

In every 9 parts, 8 consisted of siliceous sand;

the remaining (one-ninth) part was composed, in 100 parts, as follows:

Carbonate of lime (chalk) 63 grains.

Pure silex 15 grains.

Pure alumina, or the earth of clay 11 grains.

Oxide (rust) of iron 3 grains.

Vegetable and other saline matter 5 grains.

Moisture and loss 3 grains.

Thus the whole amount of organic matter in this instance is only 1 part in 200, or one-half of one per cent.; a fact which, in itself, would demonstrate the fallacy of supposing that decomposed animal and vegetable matter in the soil form the exclusive supply to growing plants.

A clayey soil, though distinguished by the color which it bears, namely black, white, yellow and red, differs from all other soils, being tough, wet, and cold, and consequently requiring a good deal of labor from the husbandman before it can be sufficiently pulverized, or placed in a state for bearing artificial crops of corn or grass. Clay land is known by the following qualities, or properties.

It holds water like a cup, and once wetted does not soon dry. In like manner, when thoroughly dry, it is not soon wetted; if we except the varieties which have a thin surface, and are the worst of all to manage. In a dry summer, clay cracks and shows a surface full of small chinks, or openings. If ploughed in a wet state, it sticks to the plough like mortar, and in a dry summer, the plough turns it up in great clods, scarcely to be broken or separated by the heaviest roller.

Clayey soils, when sufficiently enriched with manures, are naturally well qualified for carrying crops of wheat, oats, beans, and clover; but are not fitted for barley, turnips, potatoes, etc., or even for being kept under for grass longer than one year. Such soils ought to be regularly summer-fallowed once in six, or at least once in eight years, even when they are comparatively in a clean state, as they contract a sourness and adhesion from wet ploughing, only to be removed by exposure to the sun and wind during the dry months of summer. Soils of this kind receive little benefit from winter ploughing, unless so far as their surface is thereby presented to the frost, which mellows and reduces them in a manner infinitely superior to what could be accomplished by all the operations of man. Still they are not cleaned or made free of weeds by winter ploughing; and therefore this operation can only be considered as a good means for producing a seed-bed, in which the seeds of the future crop may be safely deposited. Hence the necessity of cleansing clay soils during the summer months, and of having always a large part of every clay farm under summer fallow. All clayey soils require great industry and care, as well as a considerable portion of knowledge in dressing or management to keep them in good condition; yet when their natural toughness is got the better of, they always yield the heaviest and most abundant crops. One thing requisite for a clayey soil, is to keep it rich and full of manure; a poor clay being the most ungrateful of all soils, and hardly capable of repaying the expense of labor, after being worn out and exhausted. A clayey soil also receives, comparatively, smell benefit from grass; and when once allowed to get into a sterile condition, the most active endeavors will with difficulty restore fertility to it after the lapse of many years.

Upon light soils the case is very different. These flourish under the grass husbandry; and bare summer fallow is rarely required, because they may be cleaned and cropped in the same year with that valuable esculent, turnip. Upon light soils, however, wheat can seldom be extensively cultivated; nor can a crop be obtained of equal value, either in respect to quantity or quality, as on clay sand loams. The best method of procuring wheat on light lands, is to sow upon a clover stubble, when the soil has got an artificial solidity of body and is thereby rendered capable of sustaining the grain till it arrives at maturity. The same observation applies to soils of a gravelly nature; and upon both barley is generally found of as great benefit as wheat.

Thin clays and peat earths are more friendly to the growth of oats than of other grains, though in favorable seasons a heavy crop of wheat may be obtained from a thin clayey soil, when it has been completely summer-fallowed and enriched with dung. A first application of calcareous manure is generally accompanied with great advantage upon these soils; but when once the effect of this application is over, it can hardly be repeated a second time, unless the land has been very cautiously managed after the first dressing. Neither of these soils is friendly to grass, yet there is a necessity of exercising this husbandry with them, because they are incapable of standing the plough more than a year or two in the course of a rotation.

Wheat ought to be the predominant crop upon all the rich clays and strong loams, and light soils of every kind are well qualified for turnips, barley, etc. Upon the thin and moorish soils, oats must necessarily preserve a prominent rank, and grass seeds may be cultivated upon every one of them, though with different degrees of advantage, according to the natural and artificial richness of each soil, or to the qualities which it possesses for encouraging the growth of clover, in the first instance, and preserving the roots of the plant afterwards.